1. Field of the Invention
The invention relates to a method for bending in a bending plane a multiple tube having a plurality of compartments extending side-by-side longitudinally along the tube. During the bending a bending mandrel extends into each of the compartments of the tube. The invention also relates to an apparatus for carrying out the method and to a bending mandrel for use in such apparatus.
2. Description of the Prior Art
A multiple tube means in this description and claims a tube which comprises a plurality of conduits, herein called compartments, joined together into the integral tube and extending longitudinally along the tube. The compartments are typically separated from each other by one or more partition walls. The partition wall may be a double wall, in which case each compartment is then a complete single tube in itself, or the partition wall may be a common wall between two compartments. Typically, such a tube is of metal, e.g. steel.
A known multiple tube is a so-called DD-tube which comprises two compartments, each with a D-shaped cross-section, joined to each other with the flat parts facing each other. The section of the DD-tube may be circular, elliptical or another shape. The partition wall between them is a common wall.
DD-tube is increasingly being used for exhaust pipes in the car industry. The attraction of DD-tube is that this form of tube is suitable for combustion engines with high specific power and takes up little space in a car, thus increasing the design flexibility needed e.g. for aerodynamic styling.
DD-tube is usually manufactured as straight tube with two compartments and then modified by, among other steps, a bending operation into the required shape of, for example, an exhaust pipe.
A method is known in actual practice in which DD-tube is bent in a bending plane transverse to the partition between the two D-shaped compartments. In the known method a bending mandrel is inserted into each of the D-shaped compartments. For this purpose, each bending mandrel has an outer sleeve extending with an essentially constant cross-section transverse to the longitudinal axis, this cross-section corresponds to the cross-section of the D-shaped compartment. The bending mandrel has a flat front side on the end facing towards the bend; the transition area between the outer sleeve and the flat front side is slightly rounded off. A circular bending template is used. The tube is bent round the template. Each bending mandrel is inserted far enough into the tube so that the foremost boundary of the outer sleeve of the mandrel lies approximately 2-5 mm in front of the radius through the tangent point of the tube with the bending template. The correct position of each of the bending mandrels is determined by experiment for each tube size, bending radius etc. and is thereafter not again altered. If the bending mandrel is inserted further into the tube than the above mentioned position, then a bulge occurs in the bend formed which is an impression of the said transition area. If a bending mandrel is inserted even further into the tube, then there is a risk of the bending mandrel being drawn by the tube into the bend during bending, which obstructs the bending operation and can cause damage to the bending mandrel and other parts of the bending apparatus. With the known method, bends of approximately 90.degree. may be formed, that is to say that the multiple tube is bent through 90.degree.. When a larger bend is made, the outer wall of the DD-tube at the inside of the bend displays wrinkles and the compartment at the outside of the bend will become flattened. One inconvenience arising from this is that when it is used as exhaust pipe, the gas balance between the combustion engine cylinders connected to the one D-shaped compartment and the cylinders connected to the other D-shaped compartment is disturbed.
Another disadvantage of the known method occurs when it is attempted to bend the DD-tube in two opposing bends in the same bending plane, i.e. when forming an S-shaped bend. Because the position of the bending mandrel at the inside of the bend may be different from the position of the bending mandrel at the outside of the bend, when making an S-shaped bend, the bending mandrels have to be moved from one compartment to the other during the bending operation. This results in lost production time.
U.S. Pat. No. 4,009,601 relates to a method and an apparatus for bending a double pipe, in particular two concentric round pipes. A metal core, having an outer metal core and an inner metal core is inserted into the two concentric round pipes to a prescribed position and is fixed thereon during the bending operation. The inner metal core and outer metal core are rigidly fixed relative to each other by means of a metal rod and a connecting flange.
DE-A-2 732 046 relates to a method for bending two pipes, one inserted inside the other, and to an apparatus for carrying out the method. According to this publication two mandrels are used, a round mandrel inserted inside the smaller pipe, and a crescent shaped mandrel between the smaller and the larger pipe. Both mandrels are connected to a support via rigid rods and are kept at their locations while bending the pipes.